Endangered Audio Research - Offering "The Gristleizer", which is an updated version of Throbbing Gristles wonderful audio processor. It's a bit like an LFO, a bit like an overdrive, a bit like a filter and a bit like chaos. They sell it as a kit or pre-build in several different styles (desktop, pedal & eurorack). Also makers of a top notch delay pedal.

Hex Inverter - Nice collection of PCB/kits for Eurorack as well as MU/MOTM formats

Re:Synthesis - Excellent manufacturer of custom modular panels in any format you need